What is DeFi?
Also called: decentralized finance, decentralised finance
DeFi is financial infrastructure built from public smart contracts: lending, exchange, derivatives, and stablecoins that anyone can use or build on without applying to a gatekeeper.
In practice
DeFi is the largest category in Web3 by value locked, and the least forgiving. Contracts hold real money, so audits, formal verification, and bug bounties are expected rather than optional. Builders moving into DeFi are usually assessed on security judgement first and product sense second.